1. Field of Invention
The invention relates to a fuel cell electrocatalyst and a method of producing the same, and particularly to a fuel cell electrocatalyst with an excellent carbon monoxide poisoning resistance (CO poisoning resistance), and a method of producing the same.
2. Description of Related Art
A polymer electrolyte fuel cell (PEFC) has a high power density, operates at low temperatures, and emits little exhaust gas that contains a hazardous substance, and therefore noted as an energy source of transportation means that takes the place of a conventional internal combustion engine.
The PEFC is constructed by joining an anode to one face of a solid polymer electrolyte membrane and a cathode to the other face. For example, when hydrogen as fuel is supplied to the anode and oxygen as an oxidizing agent is supplied to the cathode, the fuel is oxidized to proton at the anode while the oxygen is reduced to water at the cathode, thereby generating electric power. For both anode and cathode, a fuel cell electrocatalyst made of fine powder with a precious metal such as Pt supported on a carrier such as carbon is used.
In practice, hydrogen used in the PEFC is obtained by reforming gasoline, methane, methanol, and the like. In a reforming reaction, carbon monoxide (CO) is generated together with hydrogen. The catalyst is deteriorated by such CO (poisoned by CO), and an electric power generation voltage of the PEFC is reduced. However, it is considerably difficult to completely eliminate CO from hydrogen obtained by reforming.
It is proposed to use a Pt—Ru alloy as a catalyst that is less susceptible to an adverse effect caused by a certain amount of CO residue, and has an excellent resistance against CO poisoning. With regard to a method of producing a fuel cell electrocatalyst that carries a Pt—Ru alloy, a carrier such as carbon powder is brought into contact with a solution including positive ions of Pt and Ru to adsorb the positive ions on the carrier, and the carrier is then heated in a reducing atmosphere to reduce the positive ions. Consequently, a fuel cell electrocatalyst having the catalyst layer including a Pt—Ru alloy supported on a carrier can be produced. For example, a method of producing a catalyst by reducing metals of Pt and Ru in order in a reducing airflow of hydrogen gas or the like is disclosed (Japanese Patent Laid-Open Publication No. 9-153366).
Conventionally, various studies have been made with respect to a catalyst using a Pt—Ru alloy in order to further improve a CO poisoning resistance. For example, a proper combination of Pt and Ru is disclosed (Japanese Patent Laid-Open Publication No. 2000-12043). Moreover, a method of producing a high-performance catalyst having a high CO poisoning resistance, with a Pt—Ru alloy supported in small amounts, is disclosed (Japanese Patent Laid-Open Publication No. 2001-283867) on the basis of results of testing various combinations of positive ions including Pt and positive ions including Ru.
However, the performance of a conventional catalyst using Pt—Ru alloy was not sufficient, thus requiring further improvement of the performance. Particularly, a further improvement of the CO poisoning resistance was required when the catalyst is used as an anode catalyst for a gas-reforming type fuel cell.